The present invention relates to a tag communications system used for the electronic surveillance and tracking of articles within a defined region. In the system of the present invention, a system monitor, communicates with and tracks tags and hence the articles, which may be moved about within the region. The present invention is useful where large numbers of tags are located within the region.
Methods and communications systems for electronic surveillance and the tracking of articles within a defined region are generally known. A systems tag can be attached to a variety of objects such as parts within a warehouse, vehicles within a region, persons on a battlefield or livestock within a pasture. The monitoring device can track the objects such as inventory or stock within a warehouse, vehicle or personal location, or for other purposes necessary for locating and tracking articles.
Simple systems for tracking articles within a region may include labels affixed to the articles and bar code readers. In such a system, bar code readers can be physically brought to the tag, or the articles with the bar codes can be passed near a bar code reader. Such systems have major limitations such as locating the article once it has been warehoused. Furthermore, such a simple system would not be practical for use on a large volume of articles to be sampled.
Thus, there is a great need in the art for an efficient, low-cost accurate system that allows a central monitoring unit to communicate with tags attached to a large number of articles within a defined region. Due to the great number of articles involved, the system must be capable of remotely communicating with the tag to identify information such as location.
Systems have been developed for the remote monitoring of a plurality of tags through the use of radio frequency (RF) identification. One such system is a magnetic coupling system wherein a tag passes through a magnetic field in order to alert the monitoring system of the articles of passage through the region. A common use of such a system is the use of tags on retail merchandise which activates an alarm if a customer attempts to remove the article from the premises.
Other types of RF systems include a reflective tag which reflects the monitor's RF radiation to a receiver to identify the article. Such systems are deficient as they do not include on-board power sources. The reflective signals are inherently weak and may be limited to short range usage.
To increase the range of such RF systems, tags employ on-board power systems to allow active communication between an interrogator and a plurality of tags within the region. In such a system, an interrogator sends out a signal to a particular tag identifying the location of the tag. Such systems are not practical where a large number of articles are within the region. Furthermore, such systems may not be practical as items may remain stationary within a region for long periods of time, and it is inefficient and costly for the interrogator to attempt to communicate with tags which have not changed location.
Other RF systems include a transmit-only radio frequency identification (RFID) system. In an RFID system, the tags periodically communicate with a monitor to announce location. Such systems are not practical with large numbers of tags, as each tag would be assigned a unique time slot, and it would be time consuming and inefficient for each of the tags to communicate. Furthermore, such RFID system would only be updated for the length of the cycle time, and information may be delayed or inaccurate.
Other transmit-only RFID systems include a plurality of tags which speak to the monitor upon movement or some other event which causes the tag to send a signal. Such a system has inherent limitations as the tag cannot monitor the RF environment to see if the RF channel is clear prior to transmitting a tag signal and also, the system must operate with hundreds of tags while sharing the same RF frequency which creates an overlapping signals and creates data error.
Other systems offer an attempt to address some of the limitations of transmit-only RFID systems. Some RF transmit-only RFID systems include an on-board random signal generation circuit to prompt random transmissions to the central monitoring station. Although the randomization aids in reducing undesirable overlapping, a unacceptable amount of such overlap occurs. Randomization of the transmit time by the tag is calculated through a hardware module located within the tag. In this regard, the transmit times are random and are not influenced by outside environmental factors.
Accordingly, there is a substantial need in the art for an efficient tag monitoring system wherein the system operates with a large number of tags within a defined region and where the transmit time of the tags is calculated in a pseudo-random manner wherein outside environmental factors which are indicative of movement of the tag also contribute to the transmit time, and this reduces undesirable overlapping of signals.